This invention relates to optical transducers.
Optical displacement transducers are well known and generally employ a light source and receiver (such as provided by the ends of fibre-optic cables), and means to vary the amount of radiation falling on the receiver in accordance with displacement. The means by which the radiation is varied may employ a moveable mask with an aperture of variable size, or a neutral density filter the density of which varies along its direction of displacement. These transducers can function satisfactorily providing that the intensity of radiation falling on the receiver is not varied for any other reason. However, any change in the radiation emitted by the light source, such as, for example, caused by variations in power supply to the light source will produce erroneous displacement readings.
The effect of these variations in light intensity can be reduced by instead modulating the spectral content of the radiation in accordance with displacement. However, a problem with previous transducers of this kind is that it can be difficult to achieve a linear output since devices for producing spectral dispersion are not linear, and because wavelength decoders usually do not present a linear output response.
In another arrangement light is dispersed into its spectral components and a coded mask is moved within the spectrum to allow different parts of the spectrum to be transmittd according to the position of the mask. The radiation passed by the mask is focussed on one end of a fibre-optic cable and is dispersed again at the other end of the cable. By measuring the intensity of radiation at different parts of the spectra it is possible to determine the position of the mask. The problem, however, with such an arrangement is that it is difficult to ensure that all the radiation transmitted through the mask is focussed on the end of the receiving fibre, because the image formed will be spread over the image plane by virtue of the dispersed nature of the spectrum imaged on the mask. There is also a problem in that, in some circumstances it may not be possible to use a transparent mask where access is required to both sides of the mask.